Existing methods for liquid deposition are either limited to micron-scale resolution or larger or are slow and cumbersome direct-contact techniques. One exemplary liquid deposition method includes ink-jet printing. Ink-jet printing is an accurate, high-throughput, non-contact technique that has been used with a wide variety of fluids ranging from printer ink to electrical solder to polymers to biologically active molecules. Since ink-jet printing is intrinsically droplet-based, it is very well suited to digital control and thereby to mask-free deposition. Being a non-contact technique, ink jet deposition is inherently contamination-free. However, due to basic physical limitations, existing ink-jet printing is limited to feature sizes of about 15 μm or larger. Therefore, nanoscale printing, in the order of 1 to 999 nanometers, using existing ink-jet technology is not possible at least without sacrificing throughput.
Alternative writing techniques have emerged for depositing liquids at linewidths down to about 100 nm. Principle among these is the “dip-pen” technique of Mirkin and co-workers, which developed the concept based on fluid flow through the meniscus formed when a fluid-coated atomic force microscope (AFM) tip is brought into contact with (or near proximity to) a solid surface. However, dip-pen writing is inherently slow.
Dip-pen technique is capable of printing arrays as arrays of AFM tips can be fabricated. However, the tip spacing is dictated by the width and length of the AFM cantilevers, which typically is in the order of about 50 μm tip-to-tip spacing between cantilevers of 300 μm length. These dimensions define the minimum grid size of the writing array, meaning that each tip must still cover an area of at least 50 μm×300 μm. Moreover, there is significant overhead time required to reload the liquid coating on each tip when the “ink” is exhausted.
The dip-pen community is now attempting to incorporate ink reservoirs and microfluidic flow channels into the dip-pen microstructures in order to continually supply fluid to each tip. Whether these efforts will be successful remains to be seen. In all events, dip-pen direct writing is a cumbersome approach in comparison to ink-jet printing. However, it does offer nanoscale resolution, which current ink-jet deposition does not.
Another printing technology is photolithographic process. Broadly speaking, it is a process by which an image is transferred from a mask to a wafer through the use of a photosensitive material often called photoresist. Through light-activated chemical reaction, a photoresist is either process hardened or process softened. The process also involves etching and baking to achieve the desired outcome. However, because etching and baking are involved, the process is generally not appropriate for depositing biologically active materials, such as DNA, diagnostic immunoassay, antibody and protein arrays.
Accordingly, there is a need for nanoscale printing capable of high rate and accuracy without the drawbacks of prior art printers. A high throughput, non-contact, nanoscale printing technology would improve existing fabrication technology by decreasing device structure sizes into the nanoscale regime. In addition, a discrete droplet nanoscale printing technique would allow the use of single molecules as fundamental building blocks for fabrication, including both complex and/or biologically active molecules. Exemplary areas of technological relevance include the deposition of biomolecules (e.g., proteins, peptides, DNA) in nanoarrays for biological sensor applications and the patterned deposition of complex molecular species to form electronic and mechanical devices based on single molecules and/or on nanostructures of molecules.